The invention relates to a process for separating 2,6-dimethylnaphthalene in a high purity of more than 98 weight % from a mixture containing this methyl substituted naphthalene and a process for separating 2-methylnaphthalene in a high purity of more than 98 weight % from a mixture containing this methyl substituted naphthalene. The dimethylnaphthalene and the methylnaphthalene are hereinafter abbreviated respectively, as DMN and MN.
2,6-DMN is oxidized and 2-MN is acylated and then oxidized to produce naphthalene-2,6-dicarboxylic acid, which is an industrially important material for manufacturing polyesters and plasticizers. 2,6-DMN and 2-MN are contained in various fractions of petroleum and coal tar as mixtures together with other DMN and MN isomers.
As for separation of 2,6-DMN and 2-MN from such fractions, various processes have been proposed.
For instance, it is well known to those skilled in the art to cool the DMN fraction obtained by concentrating and extracting petroleum or coal tar material so as to obtain a solid product containing 2,6- and 2,7-DMN, which is then subjected to recrystallization or partial melting in order to separate 2,6-DMN. 2-MN is separated by continuous crystallization or recrystallization of the MN fraction in order to separate 2-MN
The DMN compounds, however, generally form eutectic mixtures. For instance, 2,6-DMN and 2,7-DMN form a two-component eutectic mixture in the mole ratio of 41.5:58.5. 2,6- and 2,3-DMN form a two-component eutectic mixture in the mole ratio of 47.5:52.5. Therefore, the conventional process for separating 2,6-DMN of a high purity which relies on the recrystallization method cannot attain a high separation yield, since the yield of 2,6-DMN is based on the material composition.
For instance the fraction of the boiling point of 250.degree.-270.degree. C. obtained by catalytically cracking petroleum contains 8-13% of 2,6-DMN and 8-13% of 2,7-DMN so that when separating and purifying by cooling, solidifying and recrystallizing or partially melting thereof, the yield for recovering of 2,6-DMN is about 30% at the highest.
It is possible to increase the 2,6-DMN content in the material up to 30% by rectification, but it is impossible to considerably change the ratio of 2,6-DMN and 2,7-DMN so the yield of pure 2,6-DMN cannot be raised.
Various fractions from petroleum or coal tar contain 2,6-DMN and 2,7-DMN in the same amount in addition to which various components inclusive of DMN isomers are contained. 2-MN and 1-MN are contained in the fraction in the ratio of 2:1. The boiling points of 2,6-DMN and 2,7-DMN as well as of 2-MN and 1-MN are very close respectively to each other so that the eutectic mixture and the solid solution thereof may be formed.
Thus, separation of 2,6-DMN encounters the problems of decreased recovery yield, difficulty of raising the purity and considerably high cost of separation and purification. The same is applied to separation and purification of 2-MN.
In order to solve the problems referred to above, utilizing crystallization under pressure, has been proposed. This method is superior to recrystallization, partial melting and continuous crystallization based on compactness of the apparatus, lower cost, higher yield and higher purity, but it is disadvantageous in that impurities in the material are subjected to oxidative polymerization with a relatively small amount of oxygen due to local superheat under high pressure in the pressure crystallizing apparatus also, the oxidized polymers become mixed into the separated product crystals.
The crystals of 2,6-DMN and 2-MN separated according to this pressure crystallization method are, thus, colored in black and the qualities thereof are considerably deteriorated due to the oxidized polyme impurities in the material so that the separated products are commercially less valuable. The method also has a disadvantage in that the discharged liquid can not be reused for the reason as referred to above.